This invention relates to a trimming mechanism for sewing machines having a trimming blade, which is forcibly moved up and down by means of the arm shaft, and a sewing machine drive controllable by means of an actuating device for driving the shaft and stopping the same in a predetermined angular position.
There are already known fabric cutting devices for sewing machines, for example from the German Pat. No. 14 85 193, the cutting knife of which is driven by the armshaft of the sewing machine by means of a lifting eccentric, so that the knife cuts at each downstroke, i.e. while the workpiece feed is inoperative. This low frequency of up and down motions of the cutting knife with regard to that of the needle (ratio 1 : 1) is sufficient to obtain properly cut workpiece edges, but only when sewing at a higher speed and when the individual cuts of the knife are formed in straight direction. During cutting of a workpiece, while sewing an outer curve having a small radius, feeding speed of the workpiece is increased at the workpiece edge with regard to the seam to be formed as a consequence of the distance between the needle and the cutting knife. Thus, in order to have the same ratio as required while sewing straight seams, the frequency of the knife motions must be higher than that of the needle motions. However, in conjunction with these known fabric cutting devices, it is impossible to cut the workpiece edge during a standstill of the sewing machine by turning the workpiece about the needle located in the workpiece in lower position, in order to change the sewing direction, since in consequence of the mechanical coupling of the cutting knife to the needle bar drive, the knife is inoperative during standstill of the sewing machine.
In conjunction with a further fabric cutting device for sewing machines known from the German Pat. No. 821 911, U.S. Pat. No. 2,533,197 the cutting knife of which is connected to a shaft causing the up and down motions of the needle, the frequency of the up and down motions of the cutting knife with regard to that of the needle are controlled by a mechanism operated by centrifugal force. Of course, this known fabric cutting device makes it possible at high sewing speed and at each revolution of the armshaft, for the cutting knife and the needle to perform one up and down motion, and, at a reduced speed, to increase the frequency of up and down motions of the cutting knife with regard to that of the needle period. However, also in conjunction with this fabric cutting device it is impossible to cut the workpiece during a standstill of the sewing machine.
Moreover, since, in conjunction with this known fabric cutting device there are arranged belt pulleys with variable diameters between the armshaft of the sewing machine and the drive shaft of the cutting knife, it is probably impossible to control the knife and the needle synchronously at high sewing speed, in order to also actuate the knife at each downstroke of the needle. However, this is required, in order to obtain a properly cut workpiece edge and a minimum wear and tear of the knife at the most favourable frequency ratio of 1 : 1 of the needle and knife motions.
It is also known from the German Pat. No. 647 023, U.S. Pat. No. 2,108,138 to drive the fabric cutting device independently of the needle by means of an electromotor with constant speed, so that the cutting knife can form cutting motions, the frequency of which is changed automatically at different sewing speeds with regard to that of the up and down motions of the needle.
Since the cutting knife is actuated independently of the sewing machine drive, this known fabric cutting device can also be actuated when the sewing machine is stopped, in order to perform the aforesaid cutting cycle when the workpiece is turned about the needle. However, the knife motions are not in synchronism with the needle. The knife also performs cutting motions during the feeding of the workpiece, so that at a ratio of frequency 1 : 1 of the knife motions to the needle motions, an accumulation of the workpiece cannot be avoided in the area of the knife, which is just cutting the workpiece. In order to obtain properly cut edges, the frequency of knife motions must be considerably higher than the knife motions forcibly imparted by the sewing machine drive. However, the high frequency of knife motions in relation to the number of up and down motions of the needle causes a reduced working life of the knife.
It is the primary object of this invention to provide an improved trimming mechanism for use in connection with a sewing machine, which is adapted to trim the lateral edges of a fabric with a minimum of up and down motions of the trimming blade.
It is a further object of this invention to provide a trimming mechanism known from the German Pat. No. 14 85 193 with means for imparting additional up and down motions to the blade when sewing at a lower speed.
Still another object of this invention is to provide the trimming mechanism with means which allow the trimming mechanism to operate during a stillstand of the sewing machine, in order to cut the edge of the workpiece while it is turning about the needle.
A further object of this invention is the provision of simple means for controlling the additional up and down motions of the trimming blade.